Solventless vacuum pressure-impregnated resins have often been employed for coil insulation of electrical generators. Although these materials have excellent electrical performance, modern turbine generators require demanding mechanical performance of the ground wall insulation to withstand higher mechanical loads encountered with longer and larger stator coils, and the like. One approach currently employed is to increase the mechanical properties of the coil insulation by using glass fabric reinforced mica tapes Such resin systems and concepts are disclosed in Smith, et al. U.S. Pat. No. 4,356,417 ('417); Rogers, Jr., et al., U.S. Pat. No. 3,868,613 ('613); Smith, U.S. Pat. No. 4,085,250 ('250); Foster, U.S. Pat. No. 4,013,987 ('987); Foster, et al., U.S. Pat. No. 3,919,348 ('348); Foster, U.S. Pat. No. 4,038,339 ('339); Smith, et al., U.S. Pat. No. 4,173,593 ('593) and Smith, et al., U.S. Pat. No. 4,254,351 ('351), all of which are hereby incorporated by reference.
Foster, et al. ('348) relates to a solventless epoxy-styrene-resin varnish comprising an epoxy resin mixture, an anhydride and a co-reactive vinyl monomer The epoxy resin can include a glycidyl polyether of a dihydric phenol. The vinyl monomer may include a styrene compound Similar epoxy-styrene-anhydride solventless impregnating compositions are disclosed in the above-identified '339; '593; and '351 patents.
Heilman, et al., U.S. Pat. No. 4,017,453, which is also incorporated by reference, relates to a thermosetting composition comprising a vinyl monomer, such as styrene, an anhydride and an epoxy. One of the disclosed diluents includes phenylglycidyl ether in quantities up to about 50% of the polyepoxide.
Hewitt, Jr., U.S. Pat. No. 4,284,753, which is also hereby incorporated by reference, relates a heat-curable epoxy-styrene blend composition This reference teaches a heat-curable composition comprising a polyepoxide, an unsaturated aromatic monomer, such as styrene, an acid anhydride, a cure accelerator composition, and a free-radical curing agent.
Although a number of epoxy systems have been disclosed, there remains a need for enhancing the mechanical properties of resins suitable for use as groundwall insulation for turbine generators. Such a resinous compositions need to have increased tensile strength, tensile modulus, tensile shear, fracture and glass transition properties.